There are many instances where it is desirable to be able to diagnose intermittent spontaneous cardiac arrhythmias in ambulatory patients. Frequently faintness, syncope, and tachyarrhythmia palpitation symptoms cannot be induced and observed by the physician in tests conducted in a clinic. For many years, such patients have been equipped with external ECG monitoring systems (i.e., the patient-worn, real time Holter monitors that continuously sample the ECG from skin electrodes and record it over a certain time period). However, the ECG data must then be analyzed to locate evidence of an arrhythmia episode from which a diagnosis can be made.
As described in commonly assigned U.S. Pat. Nos. 5,312,446 and 4,947,858, the content of which are both incorporated herein by reference, externally worn ECG recorders have inherent limitations in their memory capacity for storing sampled ECG and EGM data. Cost, size, power consumption, and the sheer volume of data over time have limited real-time external Holter monitors to recording approximately 24- or 48-hour segments or recording shorter data segments associated with arrhythmias. Typically, the patient initiates storage of a data segment after the patient feels symptoms of a possible arrhythmia. The use of an externally worn Holter monitor coupled with skin electrodes is also inconvenient and uncomfortable to the patient. The skin electrodes can work loose over time with movement by the patient, and the loose electrodes generate electrical noise that is recorded along with the ECG signal and makes the subsequent analysis difficult. It has long been desired to provide an implantable monitor or recorder that is hardly noticeable by the patient and provides the capability of recording only ECG data correlated with an arrhythmia episode that is automatically detected.
The Medtronic® Reveal® ILR is intended to be implanted subcutaneously and has a pair of sense electrodes spaced apart on the device housing that are used to pick up the subcutaneous ECG which is also characterized as a “far field EGM,” hereinafter to be generally referred to as “ECG” for brevity. The Reveal® ILR samples and records one or more segments (depending on the programmed operating mode) of such ECG signals. Such recordings occur only when the patient feels the effects of an arrhythmic episode and activates the recording function by holding a telemetry-based activator over the site of implantation and pressing a button. For example, the storage of a programmable length segment of the ECG can be initiated when the patient feels faint due to a bradycardia or tachycardia or feels the palpitations that accompany certain tachycardias. The memory capacity, however, is limited to a fixed number of patient- or automatically-activated events. Therefore, the segments of such ECG episode data that are stored in memory may be written over by newer ECG episode data when the auto-trigger storage and the auto-memory buffers are full. The stored segment or segments of episode data may be transmitted via an uplink telemetry transmission to an external programmer when the physician or medical care provider using the programmer initiates a memory interrogation telemetry session. Aspects of the Reveal® ILR are disclosed in commonly assigned PCT publication WO98/02209, incorporated herein by reference in its totality.
Monitoring long-term ECGs can help detect intermittent heart irregularities and syncopal events, among others. For example, U.S. Pat. No. 4,223,678, issued to Langer et al., discloses an arrhythmia record/playback component within an implantable defibrillator. ECG data is converted from analog to digital (A/D) form and stored in a first-in, first-out memory. When the defibrillator detects an arrhythmia event, it disables access to the memory so that no further ECG data is recorded in the memory until a command is received from an external monitoring device. This command requests the implantable defibrillator to transmit the stored ECG data to the monitoring device via telemetry. Langer et al., in U.S. Pat. No. 4,407,288, discloses a programmable, microprocessor-based implantable defibrillator that senses and loads ECG data into memory via a direct memory access operation. A processor analyzes this ECG data in the memory to detect the occurrence of an arrhythmia event. Upon detecting such an event, the defibrillator may initiate a therapy to terminate the arrhythmia and store the ECG data sequence of the event for transmission to an external monitoring device and later study. In normal circumstances, when no arrhythmia event is occurring, the defibrillator continuously overwrites the ECG data in the memory.
“Blanking” and “refractory” periods are commonly used in pacemakers and ICDs today. A blanking period is used to completely “mask” the presence of cardiac depolarizations, pacing output pulses, and ringing of the sense amplifiers. Refractory periods, on the other hand, allow the sense amplifier to detect the presence of intrinsic cardiac depolarizations and react to them, depending on whether they are atrial or ventricular. Until recently, however, their application in implantable monitoring devices that do not deliver therapy, and to ECG storage devices that automatically trigger their use in far field ECG recording has not been seen. Such blanking and refractory periods tend to eliminate or limit the sensing abilities of the implanted medical devices in which they are used. Examples of the use of such periods in therapy delivery device art include U.S. Pat. No. 5,759,196, issued to Hess, et al., U.S. Pat. No. 5,117,824, issued to Keimel et al., and U.S. Pat. No. 4,974,589, issued to Sholder. All these patents are incorporated by reference herein in their entireties. Nevertheless, none of these devices use such periods to exclude signals from the ECG. Although the terms “blanking” and “refractory” are normally used in devices that deliver therapies, their use may be extended to ILRs, since these periods have essentially the same function with regard to sensed events, which are the subject of the present invention.
Appropriate handling of noise and non-physiologic signal artifacts have always been an issue in ICDs. Various measures have been applied, though most involve the use of an automatically adjusting sensing threshold. In U.S. Pat. No. 5,381,803, Herleikson et al. disclose a method to arrive at a noise threshold which they claim allows the ICD to distinguish between R-waves and noise. A similar method is disclosed in U.S. Pat. No. 5,957,857 issued to Hartley et al. In U.S. Pat. No. 5,339,820 issued to Henry et al. a method is disclosed for changing the device's sensitivity threshold in response to the amplitude of a detected R-wave. In U.S. Pat. No. 5,658,317 to Haefner et al. a digital template is proposed for generating circuitry to help differentiate between native R-waves and repolarization waves (i.e., T-waves). Upon sensing a native event, the sensing threshold rises to a peak value and, from that point decreases in discrete steps by a defined percentage until the threshold achieves a low threshold value. A somewhat similar method is disclosed in U.S. Pat. No. 5,709,215, issued to Perttu et al. In U.S. Pat. No. 5,718,242 to McClure et al. a method is disclosed whereby the ICD uses the electrogram signal to distinguish between R-waves and noise. The ECG signal is converted into a plurality of discrete digital signals that are applied to both a cardiac event and morphology detector to control the sensitivity gain to eliminate sensing of noise.
Turning to heart monitors, Lo et al. have disclosed in U.S. Pat. Nos. 5,738,104 and 5,876,350 filtering methods to detect cardiac signals. In the '104 patent, they disclose the use of digital filtering to remove noise, followed by digital enhancement of the signal. In this way, they claim that the actual cardiac signals may be distinguished from various types of noise. The '350 patent emphasizes the use of a digital filtering mechanism to emphasize QRS complexes.
In U.S. Pat. No. 5,987,352, incorporated herein by reference in its totality, Klein et al. disclose a minimally invasive implantable cardiac monitoring device that has the capability to automatically capture arrhythmias without patient intervention. This invention also uses a fixed sensing threshold approach. Moreover, the device communicates its results via telemetry. More complex implantable monitors and pacemakers of this type, with more electrodes arranged in a planar array on the device housing, are disclosed in commonly assigned U.S. Pat. No. 5,331,966, incorporated herein by reference in its totality. This patent discloses the use of a subcutaneous multi-electrode system to detect and record ECGs. While the spacing of the electrodes may be critical, the ability to switch vectors enables the device to better discriminate between R-waves and noise. Three electrodes are employed to provide a pair of orthogonally sensed ECG signals at the subcutaneous implantation site. A medical electrical lead can be employed in a disclosed pacemaker embodiment to use a bipolar electrode pair in a heart chamber to provide an additional near field EGM sense signal. The P-wave or R-wave, depending on the location of the bipolar electrode pair, can then be sensed. Recording of the near field and ECG episode data can be invoked automatically by detection and satisfaction of bradycardia, tachyarrhythmia, or asystole detection criteria. Recording can also be manually commenced by the patient using an external limited function programmer or by the physician using a full function programmer.
In all of these implantable monitoring devices which possess a cardiac monitoring function, the cardiac ECG is continually sensed and sampled and the recording of ECG episode data is triggered in a variety of ways. Recordings of ECG episode data triggered by the patient using the relatively simple Reveal® ILR have proven to be of great value in diagnosing the causes of symptoms felt by the patients. Such devices also help when prescribing the implantation and programming of more complex therapy delivery IMDs, e.g., multi-programmable physiologic DDDR pacemakers and single and dual chamber ICDs.
However, many times patients are either unaware of symptom free (or essentially “silent”) cardiac arrhythmias, are asleep or otherwise fail to activate the recording function (e.g., following recovery from syncope) when a bradyarrhythmia and/or a tachyarrhythmia has occurred. Thus, the accompanying ECG episode data is not recorded. It is highly desirable that such devices automatically detect an arrhythmia and initiate recording of the ECG data without having to rely upon the patient as disclosed in the above-incorporated U.S. Pat. No. 5,331,966 patent. In addition, the subcutaneous location and environment of the sensing electrode pair or pairs (typically disposed on the device housing) is relatively noisy due to myopotential signals generated by adjacent muscle groups, especially during patient exercise. Limb and trunk movements or even breathing can generate noise spikes that are superimposed upon the EGM signal and can make it appear to reflect a higher heart rate than is actually present. The myopotential noise level is not as pronounced in relation to the ECG signal level when bipolar sense electrode pairs, typically located in or close to the atrium and/or ventricle, are employed as is typically the case with bipolar implantable pacemakers and ICDs. Consequently, it is usually possible to filter out such noise in the sense amplifiers of such IMDs. And, a patient implanted with a Reveal® ILR can be instructed to assume a quiet body state when he/she initiates recording. Moreover, even if noise artifacts are recorded, they may be recorded within ECG episode data that does represent an arrhythmia felt by the patient.
In this context, if an ILR of this type is implemented with an automatic arrhythmia detection function, it will automatically commence the recording of the ECG episode data when noise artifacts are superimposed on the ECG signal. In effect, the detection algorithm mistakenly detects an arrhythmia. On the other hand, sometimes such noise is present during an actual arrhythmia that is correctly detected. Unfortunately, triggering the recording of such an ECG results in a noisy and, perhaps, useless recording. Due to the limited memory capacity, the ECG data episode that is corrupted by noise signals will replace earlier recorded ECG episode data. Earlier recordings may be relatively noise free and actually represent an arrhythmia episode of interest. The physician may find that the noisy ECG data displayed by the programmer is simply too corrupted and of no value in diagnosing the patient's cardiac condition. To counteract such a problem, the physician may have to program the ILR detection algorithm differently or turn the automatic detection and recording capability off. In such cases, the physician must rely upon the patient to trigger the recording of ECG episode data when the onset of an arrhythmia is felt.
Such issues occur most often in looping-type ECG recording systems that automatically detect arrhythmias according to specific arrhythmia detection criteria and retain segments of the ECG in recorded data memories as well as in other ECG recording systems. There are several areas in which false detection events can fill up the data storage memory of a device with essentially useless data. The phrase “false detection events,” as used here, means that a predetermined number of QRS segments or R-waves has been detected over an appropriately predetermined trigger time. These set off a trigger criterion that monitors the detection of R-waves and sends the data to the trigger monitoring circuit. The trigger circuit then sets off a detect signal, forcing the implantable recorder to record a segment of the ECG into the data storage memory of the IMD.
First among likely noise sources are false detections of noise leading to false tachyarrhythmia detections; that is, inappropriate detections of an electrocardiogram segment. Muscle noise, or myopotentials, can easily dominate the ECG signal, especially when the ECG is derived from closely spaced electrodes. While it is impossible to filter out all of such noise, the circuitry is particularly susceptible to noise in the subcutaneous area where the electrodes of the small ECG implantable monitor are usually located. This noise will generally be broadband-type noise and can easily subsume the bandwidth of the standard recorded ECG. The standard recorded ECG band is a −3 dB band with ranges from 0.1 Hz to 32 Hz.
A problem caused by noise is the overreaction of the recording system such that, because of false repeat detections of the same sequence of arrhythmia, the memory overfills with segments of the same event. Such a problem is expected in an ILR ECG recording systems that automatically detects an arrhythmia and then saves data that has been monitored for a period of time previous to the detection, as well as data from a period of time following the detection. At times, this overreaction issue can be corrected by removal of noise. Then again, some of this problem can be overcome by the use of redundancy in the trigger itself. For example, a string of detected R-to-R intervals may be a condition that must be satisfied before storage of an ECG data begins.
A third noise source is electrical interference from external electrical devices in the area, commonly termed electromagnetic interference (EMI). Most common EMI is in the 60 Hz range because most alternating current and electrical devices operate in this frequency in the U.S. Any commonly available filtering techniques and digital signal processing techniques may be employed beyond what is described here for reducing this particular kind of noise.
A fourth source of false detects comes from relatively wide QRS complexes. These could include an inappropriate tachyarrhythmia detection for the occurrence of a very wide (i.e., long duration) QRS complex as are often found in patients with congestive heart failure. Such might occur if, for example, the R-wave sensing circuit had a short enough refractory period to detect multiple wavefront edges (within such a single, long duration QRS). These signals would result in multiple detections within that long QRS with the most likely number of detections being two. Thus, the R-wave detector finds several putative R-wave detections during a single QRS and, at the same time, the next QRS also has several possible R-wave detections in it. As a result, at a high enough normal rate, a very short string of such events could cause the tachyarrhythmia detection algorithm to begin recording of an episode that is not present—a false tachyarrhythmia detection.
To ensure correct discrimination between native cardiac events and such noise as described above, U.S. Pat. No. 6,236,882 issued to Lee et al., and incorporated herein by reference in its totality, discloses a looping-type memory to store cardiac events. Senses that occur within a blanking period are not detected or counted, whereas those that occur during the subsequent refractory period are not added to the trigger count that could indicate the presence of an arrhythmia. In fact, these events might cause the resetting of the trigger count. As previously discussed above in reference to U.S. Pat. Nos. 5,759,196, 5,117,824, and 4,974,589, the inventors of this patent have elected to use “blanking” and “refractory” rather than “denial” and “accommodation” intervals as used in the '882 patent.
Duffin, in U.S. Pat. No. 6,230,059, also incorporated herein by reference in its totality, teaches a method to screen out and discriminate noise from intrinsic R-waves. The device then records the noisy and relatively noise free segments of physiologic data in separate memory registers of a limited memory for retrieval and analysis at a later time.
Although both of the above patents are able to discriminate between R-waves and noise better than previous patents, there are still ways in which such discrimination can be improved, in particular, the handling of P- and T-waves and the sudden variations of R-wave amplitudes. The present invention provides novel means for such improvements.